This invention relates to remote sensing techniques to detect gas leaks. In particular, the invention involves flying an aircraft with a remote sensing instrument over a target area, such as a pipeline, and measuring absorption of upwelling electromagnetic radiation that has passed through natural gas.
Past attempts to remotely detect natural gas leaks have involved detecting increased concentrations of methane (CH4). CH4 comprises approximately 95% of the composition of natural gas, which makes it a natural target for detection. One problem that has been experienced is that CH4 exists in fairly large quantities in the atmosphere (it is well mixed in the atmosphere with a concentration of approximately 1.7 ppm). Therefore, detecting a gas leak required detection of a small increase on a large background. Events such as passing near a source region of CH4 (such as a farm), or an increase in the altitude of the airplane (an increase in the atmospheric path length) might result in the false signature of a leak.
To reduce the influence of the background, some past attempts have tried to detect the excess CH4 of a natural gas leak by detecting the absorption of CH4 in the long wavelength infrared region (for example, at 7.8 μm or 2180 cm−1). This provides the advantage that the upwelling radiation is primarily emitted from the earth's surface. This minimises the background CH4, as only the CH4 located between the airplane and the earth's surface is detected.
However, for underground pipe since the temperature of the surface and the leaked CH4 are nearly the same, the radiative contrast between the surface and the leaked methane is very small, greatly reducing the detectivity/detectability of the leak. Also, the thermal noise introduced within the instrument itself becomes a serious design constraint. Using a shorter wavelength absorption band of CH4 could potentially help, as the upwelling radiation would be primarily from the sun. This would greatly increase the radiative contrast between the source and the “leaked” gas, and significantly reduce the thermal noise within the instrument. However, the background of CH4 becomes very large, as the solar radiation reaching the instrument would have passed through entire atmosphere.